Operational transconductance amplifiers can be used, for example, in any switched capacitor (SC) circuit, in particular in switched capacitor filters or in sigma delta analog/digital converters.
In FIG. 2 as an example a cascode operational transconductance amplifier is illustrated in accordance with the state of the art, whereby the example illustrated in FIG. 2 concerns in particular a fully differential “Folded cascode operational transconductance amplifier”.
The operational transconductance amplifier shown in FIG. 2 comprises an input stage 10 with two transistors M1, M2, which form a differential pair. The gate terminals of the two differential pair transistors M1, M2 correspond to input terminals inp and inn, respectively, to which differential input signals are fed. The drain terminals of the differential pair transistors M1, M2 are linked with one another and connected to a serial circuit consisting of two further transistors M11, M12, which form a cascode circuit. The transistors M11, M12, through which a current Iss flows, thus form a power source for the differential pair transistors M1 and M2.
An output stage 20 is coupled with the input stage 10, said output stage comprising two output signal paths with a transistor M5 and M6, respectively, in each case serving as an amplifier element. The differential output signals corresponding to the amplified differential input signals can be picked up on the source terminals of these two transistors M5 and M6 via output signals outp and outn, respectively, since the drain terminals of the transistors M5 and M6 are connected to the source terminal of one of the differential pair transistors M1, M2. In addition, each output signal path has another transistor M3 and M4, respectively, whose source terminal is connected to the drain terminal of the transistor M5 and M6, respectively. The drain terminals of these two transistors M3, M4 are connected to a positive supply voltage VDD, so that these two transistors M3, M4 function as a power source for the transistors M5 and M6, respectively. The source terminals of these two transistors M5 and M6 are again connected in each case to a series circuit consisting of transistors M9, M7 and M11, M8, respectively, via which the transistors M5 and M6, respectively, are connected to earth. The transistors M9, M7 and M10, M8 in each case form a cascode circuit and function as a power source for the transistors M5 and M6, respectively. With the example shown in FIG. 2, the gate terminals of the transistors M3 and M4, the transistors M5 and M6, the transistors M9 and M10 as well as the transistors M7 and M8 in each case are connected to one another and biased by an appropriate bias or pre-voltage Vbias1 . . . Vbias4. Furthermore, the bias voltage Vbias3 . . . and/or Vbias4 also lies on the gate terminal of the transistor M11 and, respectively, on the gate terminal of the transistor M12.
In the case of the example illustrated in FIG. 2, the differential pair transistors M1, M2 as well as the transistors M7–M12 concern NMOS transistors, while the transistors M3–M6 concern PMOS transistors.
As already previously mentioned, such operational transconductance amplifiers are frequently used in sigma delta analog/digital converters, which are configured in accordance with the so-called switched capacitor technology. In this case, there is frequently a need to switch the sigma delta analog/digital converter for a particular operating mode to a higher clock frequency in comparison to the normal operating mode without changing the dynamic behaviour of the entire sigma delta analog/digital converter. Generally, such a problem can basically arise with any switched capacitor circuit.
In the case of the known operational transconductance amplifier circuit shown in FIG. 2, up to now the operating clock frequencies were switched over via changes in the bias voltage. For application in a sigma delta analog/digital converter it is necessary to switch over via the clock frequency in a wide range. When programming the particular operational transconductance amplifier via the bias voltages, however, this is not possible without reductions in performance. In this case, the dynamic characteristics, such as in particular the transit frequency and the phase reserve, of the operational transconductance amplifier would change too much. A further disadvantage when programming the operational transconductance amplifier via the bias voltages is that when switching over to the operating mode with the higher clock frequency the dynamic range of the operational transconductance amplifier is also affected. This results in greater nonlinear distortions, which in most applications are not desirable. Precisely with low voltage applications, as particularly in a sigma delta analog/digital converter, a change in the dynamic range of the operational transconductance amplifier is very disturbing.
The underlying object of the present invention is therefore to provide an amplifier circuit, which in the simplest way possible enables operation with various clock frequencies without changing the essential dynamic characteristics.